In the past, in order to prepare a sheet-form material, a resin composition was cured to the B stage by increasing the viscosity thereof. For curing it to the B stage, various methods in which a metal oxide such as magnesium oxide is used as a viscosity improver, in which the viscosity is increased by isocyanate, a photopolymerization initiator causing partial polymerization, or a crystalized polyester, are known.
1) Specific examples of photocurable materials are disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 63-186744, and Japanese Unexamined Patent Application, First Publication No. Hei 3-106942, in which magnesium oxide is used.
2) A sheet-form material in which isocyanate is used is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Sho 60-123538.
3) Specific examples of methods of forming into sheets by partial photopolymerization by ultraviolet irradiation are disclosed in Japanese Examined Patent Application, Second Publication No. Sho 58-21927; Japanese Unexamined Patent Application, First Publication No. Sho 56-139535; and Japanese Unexamined Patent Application, First Publication No. Sho 57-111322.
4) Specific examples of methods of increasing the viscosity by using crystalized polyesters are disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 58-67709, and Japanese Unexamined Patent Application, First Publication No. Hei 5-1123.
5) Specific examples of viscosity improvers containing fine acrylic resin powders which have recently been researched, although they are not photocurable materials, include viscosity improvers containing fine acrylic powders of unsaturated polyester resins disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 5-171022; Japanese Unexamined Patent Application, First Publication No. Hei 9-95519; Japanese Unexamined Patent Application, First Publication No. Hei 9-174698; Japanese Unexamined Patent Application, First Publication No. Hei 9-174781; Japanese Unexamined Patent Application, First Publication No. Hei 9-176331. Specific examples of acrylic resins are disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 5-32720; Japanese Unexamined Patent Application, First Publication No. Hei 5-171022; Japanese Unexamined Patent Application, First Publication No. Hei 6-298883; Japanese Unexamined Patent Application, First Publication No. Hei 6-313019; Japanese Unexamined Patent Application, First Publication No. Hei 7-188505; Japanese Unexamined Patent Application, First Publication No. Hei 8-225705; Japanese Unexamined Patent Application, First Publication No. Hei 10-67906; Japanese Unexamined Patent Application, First Publication No. Hei 10-265639; Japanese Unexamined Patent Application, First Publication No. Hei 10-279765; Japanese Unexamined Patent Application, First Publication No. Hei 10-287716.
These are used for the purpose of improving handleability, formability, low contractility, preventablity of cracks in molding, appearance, stability of viscosity increase, and kneading properties.
In accordance with the conventional methods of increasing the viscosity, when metal oxide is used, for example, metal oxide is usually used in a dispersed form because metal oxide is barely dissolved in styrene, acrylic monomer, or the like. Therefore, a resin composition becomes cloudy, and lacks transparency, and it is inferior in photocurability as a photocurable material. Moreover, since a reactive OH group or COOH group is required in a polymer component to increase the viscosity, metal oxide cannot be applied to resins not having COOH groups such as vinyl ester or the like. Since the efficiency of the viscosity increase is suppressed by water, it is difficult to control the viscosity increase to produce stable sheet-form materials.
When isocyanate is used to increase the viscosity, although it has high solubility, the efficiency of the viscosity increase is more suppressed by water than metal oxide, so it is difficult to control the increase in the viscosity. Moreover, if isocyanate remains in a sheet-form material, it tends to cause irritation to the skin, so that it is not superior in handleability.
When the viscosity is increased by photopolymerization, since light is used for final curing, it is difficult to control the degrees to which the viscosity will increase and what the hardness will be, and the result tends to lack preservation stability.
When acrylic resin powders disclosed Japanese Unexamined Patent Application, First Publication No. Hei 5-171022, for example, are used to increase the viscosity, an obtained sheet-form material becomes cloudy in portions where the acrylic resin powders remain insoluble, and the transparency of the sheet-form material is inferior, so that the photocuring efficiency is decreased.